1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to an apparatus and method for reducing the peak-to-average power ratio (PAPR) of a base station (BS) in a mobile communication system.
2. Description of the Related Art
As is known, a BS uses an RF (Radio Frequency) power amplifier for amplifying an RF signal including voice and data destined for a mobile station (MS). The RF amplifier is the most expensive device in the entire system and thus a significant component to be considered to reduce system cost. This RF amplifier should be designed to meet two requirements: one is to output RF power at a level strong enough to cover all MSs within the service area of a cell; and the other is to maintain ACI (Adjacent Channel Interference) with the output of the RF power amplifier at or below an acceptable level.
If input power that ensures sufficient RF output power is outside a linear amplification area of a power amplifier, the output signal of the power amplifier has a signal distortion component outside the signal frequency band due to non-linear amplification. In the frequency plane, in other words, spectral regrowth outside the signal frequency band causes ACI. It is very difficult to design a power amplifier satisfying these requirements because the former requires high input power and the latter requires low input power.
Especially, a system having a high PAPR such as CDMA (Code Division Multiple Access) must control the input power to enable the power amplifier to operate in the linear amplification area, or use an expensive power amplifier having linearity at maximum input power. In this context, the CDMA system needs an expensive power amplifier that can accommodate a maximum input power 10 dB higher than an average input power to suppress signal distortion. As stated above, however, such a power amplifier decreases power efficiency and increases power consumption, system size, and cost. Moreover, the BS transmits signals with a plurality of frequency allocations (FAs) at the same time using a power amplifier for each FA, thus imposing economic constraints. Therefore, efficient layout and design of power amplifiers is very significant to the design of BS.
One approach to stably operating a power amplifier in the high PAPR system is to use a pre-distortion adjusting circuit for maximum power input. The pre-distortion adjusting circuit measures signal distortion produced in the power amplifier and controls the input signal of the power amplifier based on the measurement. The power amplifier generates an amplified signal from the original input signal by attenuating the distortion.
A very complicated process is involved with the distortion measurement, such as modulation and demodulation, sampling, quantization, synchronization, and comparison between input and output. The pre-distortion adjusting circuit utilizes its input and output signals to meet ACP (Adjacent Channel Power) standards for system implementation. However, optimum distortion compensation cannot be achieved with this pre-distortion adjusting circuit due to its shortcomings associated with efficiency, speed, and complexity.
Another approach is to reduce the PAPR of an input signal in the power amplifier by decreasing the level of the signal at a predetermined rate using maximum input power and the linear amplification characteristics of the power amplifier. All input signals are converted to low power signals by multiplying them by scale factors based on the linear amplification characteristics in order to operate the power amplifier within the linear amplification area. Or the PAPR can be reduced by decreasing the power of an input signal at or above a threshold to an intended level. The decrease of the signal level at a predetermined rate or the decrease of a signal level greater than a threshold to a predetermined level results in drastic changes in the signal level and a power increase outside the signal frequency band. Consequently, the overall system performance is deteriorated.
A third approach is to calculate the strength and power of an I channel input signal and a Q channel input signal and generate cancellation signals for signals having strengths at or above thresholds. The signal strengths are reduced to a desired level by adding the original signals and the cancellation signals at the same time. Signal transmission using this amplification scheme is illustrated in FIG. 1.
Referring to FIG. 1, each channel device or channel element 1-2 in a channel device group 1-1 generates a baseband signal by subjecting input channel data to appropriate encoding, modulation and channelization in a CDMA communication system. The I and Q channel baseband signals are summed separately. A processor 1-5 measures the strengths of the I and Q channel signals, calculates their power levels, decides the strength of a signal to be removed for each channel according to a desired power level, and outputs cancellation signals. An I baseband combiner 1-3 and a Q baseband combiner 1-4 delay the I and Q channel signals by time required for the operation of the processor 1-5 and add the delayed I and Q channel signals to the cancellation signals to achieve signals at the intended power level. Pulse shaping filters 1-6 and 1-7 limit the bandwidths of the output signals of the I and Q baseband combiners 1-3 and 1-4. The outputs of the pulse shaping filters 1-6 and 1-7 are transmitted to an antenna through a frequency converter 1-8 and a power amplifier 1-9. The antenna radiates the transmission power of the BS to the MSs within its cell.
Although the PAPRs of the signals are adjusted to a desired value in the I and Q baseband combiner s 1-3 and 1-4, they increase in the pulse shaping filters 1-6 and 1-7. As a result, spectral regrowth outside the signal frequency band occurs in the power amplifier 1-9, thus causing ACI.